xDSL modems, such as ADSL, HDSL, SDSL and VDSL, among others, are well-known in the prior art. In such modems, a signal is sent over a twisted pair communication line which links a first transceiver to a second transceiver. A pair of xDSL modems arranged to communicate with each other use at least one established communication protocol. One common communication protocol, found in ADSL and VDSL modems, is discrete multitone (DMT) modulation.
In DMT modulation, an xDSL transmitter typically takes a complex-valued signal and places it for transmission in a DMT bin. The signal's coordinates in the complex plane signify the information that is to be sent to the receiver. Thus, in a typical protocol, one encodes two bits (00, 01, 10 or 11) per frequency bin using the complex plane. In general, a collection of such signals, typically called a symbol, are simultaneously sent over a predetermined number of frequency bins, typically 128, 256 or 512, depending on the standard. The symbol can be sent downstream (from the ATU-C to the ATU-R) using a higher range of frequencies, or sent upstream (from the ATU-R to the ATU-C) using a lower range of frequencies.
In a given XDSL session between a pair of modems, the different frequency bins may experience different line noise levels. The noise level may depend on such factors as the crosstalk from other twisted pairs in a cable binder, and far end modem internal noise leakage, among other things, while the received signal level may depend on the length of the twisted pair transmission line. In the typical case, most frequency bins will have a signal-to-noise ratio that is sufficiently high to permit them to be used to transmit data. Other frequency bins, however, may have SNRs that are too low and so these frequency bins may not be used. These unused frequency bins represent bandwidth that is lost, and so it is generally recognized that the number of unused bins should be kept to a minimum, subject to maintaining good signal quality.
FIG. 1 shows a typical xDSL communication system 100 showing a central office modem 102, commonly designated CO or ATU-C (ADSL transmission unit-Central), connected to a remote customer premises equipment modem 104, commonly designated CPE or ATU-R (ADSL transmission unit-remote). The two modems 102, 104 are connected by a twisted pair transmission line 106, typically formed from copper or other conductor. The length of the twisted pair may vary, but is typically on the order of less than 20,000 feet, the length being dictated by the signal level transmitted from the far end transmitter, the cable attenuation of the transmitted signal, and the level of noise at the receiver Usually, in xDSL systems, one speaks of the “loop reach”, which expresses the allowable separation between the ATU-C and the ATU-R at various data transmission rates, e.g., 12,000 ft@ 1 Mbit/sec, 14,000 feet@ 900 Kbit/sec). Longer loop reach generally means that one can serve more customers, and so it is considered to be desirable to extend the loop reach as much as possible, while still maintaining good data rates.
Although modem 102 and modem 104 may have some differences due to the nature of their roles, one at ATU-C and the other at ATU-R, they have many characteristics and capabilities in common. Both modems have EMI and safety circuitry 110a, 110b, line transformer and associated filters 112a, 112b, and a hybrid circuit 114a, 114b which couples the two wires' differential mode signal from the twisted pair to the four wires (two each for the transmitter circuitry and the receiver circuitry). In addition, each has receiver circuitry 116a, 116b comprising one or more amplifiers and filters, and also transmitter circuitry 118a, 118b, also comprising one or more amplifiers and filters.
The transmitter circuitry typically includes a line driver and filter. The filtering is the combination of analog and digital frequency/time domain shaping. The filtering limits the energy contained in the regions above and below the transmitter pass band frequencies. In the case of FDM ADSL located at the remote site ATU-R, the low pass transmitter filtering limits the upstream generated signal energy which falls into the same frequency spectrum as the downstream receive spectrum. The low pass filtering does not limit the harmonic and inter-modulation distortion generated by the line driver which falls into the receive bandwidth. The upstream signal is transferred to the twisted pair interface via the hybrid. The hybrid is a 2-wire to 4-wire converter in which the 2-wire twisted pair transmission line interface is converted to a 2-wire receiver interface and a 2-wire transmitter interface.
In most modems, some transmitter energy couples from the transmitter into the receiver via the hybrid. FIG. 2 shows a hypothetical channel for an ATU-R in which bins 6-29 are used for transmitting and bins 37-127 are used for receiving. The bins between 29 and 37 are not used because of the out-of-band energy 120 from the transmitter. Thus, these bins form a guard band, which represents unused bandwidth, and it is generally recognized that it is advantageous to reduce the width of this guard band to the extent possible. As also seen in FIG. 2, despite the presence of the guard band, some of the transmitter energy may still leak into the bins used by the ATU-R receiver. The amount of reduction of this energy between the transmitter output port and the receiver input port of the hybrid is known as trans-hybrid loss.
In the design of ADSL transceivers, it is generally preferred that the hybrid circuitry minimize the local transmitter energy that couples into the local receiver. Minimizing local transmitter energy into the local receiver reduces the dynamic range that the receiver must handle and improves the signal-to-interference (SNI) ratio. This results in an analog front end (AFE) requiring fewer analog-to-digital (ADC) bits in order to recover the desired signal from the far end transmitter, and also provides greater data carrying capacity for the end user. While minimizing this energy coupling, it is also desirable that the hybrid circuitry operate at the best impedance match possible to the twisted pair transmission line. The impedance presented at the line interface of an ADSL transceiver is a function of the twisted pair makeup and topology. This impedance can vary widely from loop to loop and a designer has no control over the range which a transceiver may need to operate.
The hybrid is typically a simple resistor, capacitor and inductor circuitry, normally fixed in value. As such, the trans-hybrid loss is a function of the termination impedance mismatch between the modem and the twister pair transmission line.
Typical values range from a 6 dB trans-hybrid loss for a poorly matched condition to 40 dB for a well matched condition. A wide range of termination impedances are provided to a modem by the twisted pair transmission line due to varying cable gauges, bridged taps, and other variable characteristics. As such, for a worst-case condition in a modem compliant with the G.992.1 or G.992.2 standard, it is possible for the upstream signal to couple into the receiver at a level as high as about +7 dBm. On an 18 kfeet loop (equivalent 26 AWG twisted pair) the received signal power at the twisted pair line interface would be only −60 dBm. The wide difference between undesired upstream echo power and desired downstream receive signal power requires system and hardware level tradeoff choices. Typically, a system level choice is made to insert a guard band between upstream and downstream signals. The hardware level choices include selection of line driver linearity, receive amplifier linearity, receive filtering levels relative to A/D converter requirements and receive filtering budgets with regards to receive amplifier linearity and low noise modem performance. The designer can trade line data capacity performance points against implementation costs and topologies.
FIG. 3 shows a prior art hybrid circuit. In FIG. 3, resistors R13 through R20 form a typical hybrid, or 2-wire to 4-wire converter. Of these, R16 and R19 are represent the line termination impedances, and are shown here as single resistors. In practice, however, these are frequently implemented as reactive terminations, i.e., combinations of Rs, Ls and Cs.
Much of a modem's communication functions are under the control of a signal processor. These communication functions may include such things as modulating and demodulating signals, echo cancellation, clipping mitigation, and filtering, among others. Thus, the signal processor is used to convert the transmitted and received digital signals from one form to another. The signal processor is typically implemented through a combination of hardware and executable software code. In the usual case, the signal processor includes a programmable computer, perhaps implemented as a reduced instruction set (RISC) computer, which handles only a handful of specific tasks. The computer is typically provided with at least one computer readable medium, such as a PROM, flash, or other non-volatile memory to store firmware and executable software code, and will usually also have an associated RAM or other volatile memory to provide workspace for data and additional software.
In the typical xDSL communication system, the signals handled by the signal processor are discrete multitone signals (DMTs) comprising N discrete tones simultaneously carried over the twisted pair. The collection of discrete tones is commonly referred to as a symbol, and a sequence of such symbols, spaced apart in time by a cyclic prefix, are transmitted in xDSL communications. A more detailed description of xDSL communication, xDSL transceivers and equalizers can be found in U.S. Pat. Nos. 5,285,474 and 5,479,447, both to Chow et al., whose contents are incorporated by reference to the extent necessary to understand the present invention.